1. Field of the Invention
The present invention relates to the combustion chamber of a direct-injection internal-combustion engine in which fuel is injected directly into the combustion chambers and, more specifically, to a combustion chamber for an internal-combustion engine, capable of combusting a volatile fuel having a low cetane number, such as light oil, gasoline and alcohol, over the entire range of loaded operation of the internal-combustion engine including the starting period, and capable of reducing the discharge of hydeocarbons (HCs), nitrogen oxides (NOx), incompletely combusted fuel and smoke, combustion noise and vibration of the internal-combustion engine without reducing the output of the internal-combustion engine and deteriorating the fuel consumption rate.
2. Description of the Prior Art
Generally, the diesel engine, in which fuel is injected directly into the combustion chamber for spontaneous ingition, has advantages that the engine operates at high thermal efficinency and the temperature of the exhaust gas is low. On the other hand, the diesel engine still has the drawback that the delay in ignition timing enhances the internal pressure of the cylinder, combustion noise and vibration.
The MAN-M engine has been developed to eliminate such drawbacks of the conventional diesel engine. In the MAN-M engine, fuel is caused to impinge on the inner surface of a spherical combustion chamber defined by the cylinder head and a piston having a concave top surface by producing a swirl of fuel in the combustion chamber so that the vapor of the fuel and air are mixed well to produce an inflammable premixture, which generally enables slow combustion.
However, since the MAN-M engine makes the fuel injected into the cylinder evaporate for combustion, the cylinder wall is not sufficiently capable of evaporating the fuel when the temperature of the atmosphere and the cooling water is low. When the fuel is not satisfactorily evaporated, much unburned substances such as HCs and white smoke are produced in the combustion chamber. The cylinder wall is not heated to a temperature high enough to evaporate the fuel during low-load operation of the engine, such as idling, even when the temperature of the atmosphere and the cooling water is relatively high, and thereby much unburned substances (HCs) are produced to deteriorate the combustion.
Such a drawback is found also in a troidal combustion chamber in which a lean mixture is combusted. Such an unsatisfactory combustion occurs in a troidal combustion chamber because the flame is unable to propagate normally after ignition and is blown out.
In order to eliminate such drawbacks, combustion chambers A, B and C have been proposed in Japanese provisional patent Publication No. 49-50307, Japanese Provisional Utility Model Publication No. 57-33221 and Japanese Provisional Patent Publication No. 57-41417, respectively. These prior art combustion chambers will be described hereunder.
As illustrated in FIG. 23 of Japanese Provisional Patent Publications No. 49-50307, the combustion chamber A is of the MAN-M fashion having a main combustion chamber 50 defined by the cylinder head and a deep recess formed in the crown 2a of a piston, a small substantially spherical precombustion chamber 52 formed in the cylinder head 51, and a connecting passage 53 formed in the cy1inder head 51 so as to interconnect the main combustion chamber 50 and the precombustion chamber 52.
As illustrated in FIG. 24 of Japanese Provisional Utility Model Publication No. 57-33221, the combustion chamber B, similar to the combustion chamber A, has a main combustion chamber 50a formed in the crown of a piston 1b and a precombustion chamber 52b formed in the cylinder head 51a. A connecting passage 53a is formed so as to interconnect the main combustion chamber 50a and the precombustion chamber 52b tangentially and to form a swirl chamber 54 when the piston 1b is at the top dead point.
In the combustion chambers A and B, fuel is injected into the precombustion chamber 52 during the low-load operation of the engine and to inject fuel into the main combustion chamber 50 during the high-load operation of the engine and, during the low-load operation of the engine in particular, the air fuel ratio (air/fuel) is reduced to suppress the production of unburned substances (HCs).
However, since the combustion chambers A and B are formed so that fuel in injected into the main combustion chamber 50 through the connecting passage 53 during the high-load operation of the engine, the injected fuel is ignited by the combustion gas prevailing within the precombustion chamber 52 before being injected into the main combustion chamber 50, and hence the spray of the injected fuel is unable to reach the main combustion chamber 50. Consequently, incomplete combustion of the fuel results in the main combustion chamber producing smoke and causing the reduction of combustion efficiency.
The combustion chamber C as illustrated in FIG. 25 of Japanese Provision Patent Publication No. 57-41417 has been proposed to eliminate the drawbacks of the combustion chambers A and B. According to this prior art chamber, a cavity 55 for receiving swirling intake air S is formed at least in either a piston 1c or a cylinder head 51b, an auxiliary swirl chamber 57 communicating with the cavity 55 by means of a swirl producing restrictive orifice 56 is formed in the crown of the piston 1c, and a fuel injection nozzle 58 is disposed so as to spray a part of the injected fuel on the inner surface 55a of the cavity 55 and to inject the rest throught the restrictive orifice into the auxiliary swirl chamber 57, so that the fuel is subjected to rapid combustion in the auxiliary swirl chamber 57 and to evaporation and slow combustion in the cavity 55. The restrictive orifice 56 opens into the cavity 55 along the swirling direction of the swirling intake air S. Thus, the combustion chamber of an internal-combustion engine has an auxiliary restrictive orifice opening in the direction of fuel injection and a main restrictive orifice opening into the cavity 55 in the swirling direction of the swirling intake air S. Part of the combustion chamber C is formed by the auxiliary swirl chamber in which a swirl is produced during the compression stroke of the piston and a portion of the injected fuel is combusted rapidly in the auxiliary swirl chamber, while the rest of the fuel is sprayed on the inner surface 55a of the cavity 55. A combustion gas produced by the rapid combustion of the fuel in the auxiliary swirl chamber 57 spouts from the auxiliary swirl chamber 57 into the cavity 55 to produce a strong turbulent flow of air in the cavity 55, so that the fuel adhering to inner surface 55a of the cavity 55 is spread in thin films to promote the evaporation of the fuel for slow combustion.
This combustion chamber C, however, has a problem that the fuel is evaporated excessively by the rapid flow of the combustion gas produced in the auxiliary swirl chamber 57 and by the heat of the combustion gas. This excessive evaporation of the fuel tends to be enhanced in the high-load operation of the engine. The promotion of the evaporation of the fuel in the cavity 55 produces a great amount of very inflammable premixture. When this premixture is ignited in the cavity 55 by the combustion gas, rapid combustion occurs in the cavity 55, which causes the internal pressure of the combustion chamber to rise sharply, and thereby noise increases abnormally.
A laminar fuel feed engine capable of using low-grade fuel, such as methanol (methyl alcohol), is disclosed in Japanese Provisional Utility Model Publication 57-107820. As shown in FIG. 26 of this publication, in this engine, a precombustion chamber 61 is formed on one side of the upper end of a cylinder 60, a preheating plug 63 is disposed transversely in the bottom wall 62 of the precombustion chamber 61, a fuel injection nozzle 64 is disposed opposite to the preheating plug 63, and an ignition plug 65 is disposed between the preheating plug 63 and the fuel injection nozzle 64.
This proposal is intended to ignite the fuel in a high-temperature atmosphere by heating the precombustion chamber with the preheating plug so that the low-load operation characteristics at the start of the engine is improved.
However, since the precombustion chamber 61 and the main combustion chamber 67 are interconnected by a throat 66, the forced flow of compressed air into the precombustion chamber 61 and the flow of the combustion gas from the precombustion chamber 61 into the main combustion chamber 67 inevitably entail a large loss due to the restrictive action of the throat, and thereby the thermal efficiency of the engine is reduced. Since the reduction of the thermal efficiency entails the reduction of the combustion temperature, reduction in the output of the engine and increase in unburned substances such as HCs are possible. Furthermore, in starting the engine at a very lowe atmospheric temperature, such as a temperature below -20.degree. C., since the fuel injection rate is increased and a fuel oil having a low heat of vaporization, such as methanol, is used, it is difficult to vaporize the fuel oil. Accordingly, a long preheating time is required or increase in the heating capacity of the preheating plug is necessary, which is undesirable from the view point of the capacity of the battery and time required for starting the engine.